Method for operating an internal combustion engine comprising at least one working piston

ABSTRACT

A method for operating an internal combustion engine with at least one working piston with a variable compression ratio, which can be changed as a function of an operating state of the internal combustion engine, the method including the steps of: detecting, with a knock sensor, when the compression ratio is too high; reducing the compression ratio (V) when the knock limit (L) is at least reached by a first, predetermined value (V 1 ), which is a function of the operating state, over a first, predetermined time period (t 1 -t 0 ); subsequently increasing the compression ratio (V) by a second, predetermined value (V 2 ) over a second, predetermined time period (t 2 -t 1 ) until the knock limit (L) is at least reached again; and determining at least one of the first predetermined value (V 1 ), the first time period (t 1 -t 0 ), the second predetermined value (V 2 ), and the second time period (t 2 -t 1 ) as a function of load demand on the internal combustion engine.

PRIORITY CLAIM

This is a U.S. national stage of application No. PCT/EP00/09281, filedon Sep. 22, 2000. Priority is claimed on that application and on thefollowing application:

Country: Germany, Application No.: 199 50 682.5, Filed: Oct. 21, 1999.

BACKGROUND OF THE INVENTION

The invention pertains to a method for operating an internal combustionengine with at least one working piston with a variable compressionratio.

A device by means of which the closing point of the intake valve of aninternal combustion engine can be varied, so that the compression ratiocan be varied as a function of the operating state of the internalcombustion engine, is known from DE 4,108,454 C2. A knock sensor, whichis mounted on the engine housing in the known internal combustionengine, is provided to detect when the compression ratio is too high.When the knock sensor responds, thus signaling that the compressionratio is too high, an adjusting mechanism changes the closing time ofthe intake valve. When the operating state changes again in such waythat it is possible for a higher compression to be used, provision ismade in the known internal combustion engine for detecting thetemperature of the wall of the combustion chamber. When this temperatureis below a predetermined value, the closing time of the intake valve canbe changed, as a result of which the compression ratio is increasedagain. The disadvantage of this internal combustion engine is that theopen-loop or closed-loop control system functions relatively slowly,especially when the compression ratio is to be increased, because thetemperature of the wall of the combustion chamber changes relativelyslowly, i.e., the temperature detection process is relatively slow.

A method for operating a four-cycle internal combustion engine is knownfrom DE 198-04,988 C1. The compression ratio is varied by changing theopening and closing time of the intake and/or outlet valve. So that thecompression ratio can be adjusted to the optimum value, various sensorsare provided to monitor the combustion process. For example, an ioncurrent sensor is provided inside the combustion chamber. In addition, aknock sensor, designed as a structure-borne sound sensor, is attached tothe engine housing. It is found that, because of the large number ofsensors, especially in the case of multi-cylinder engines, it isnecessary to process or evaluate large amounts of sensor data. Theelectronic circuitry, i.e., the control mechanism, must therefore berelatively complicated.

It is therefore the task of the invention to provide a method foroperating an internal combustion engine with at least one workingpiston, which method does not suffer from the disadvantages mentionedabove.

This task is accomplished by a method for operating an internalcombustion engine with at least one working piston, which method has thefeatures stated in claim 1. This internal combustion engine has avariable compression ratio, which can be changed as a function of theoperating state of the internal combustion engine, a knock sensor beingused to detect when the compression ratio is too high, so that thecompression ratio can then be lowered below the knock limit. Accordingto the invention, the process is characterized in that, when the knocklimit is reached or exceeded, the compression ratio is reduced by afirst, predetermined value, which is a function of the operating state,over a first, predetermined time period, and in that the compressionratio is then increased by a second, predetermined value over a second,predetermined period of time until it reaches or exceeds the knocklimit. The decrease in the compression by the first value and/or theincrease in the compression by the second value preferably occurs in acontinuously falling manner. A graphic depiction on a two-axisorthogonal coordinate system, in which the compression ratio is plottedversus time, yields a graph of the compression ratio in the form of asawtooth-like curve. The compression ratio is lowered and then raisedagain as a function of time by specific, predetermined values, whichdepend on the operating state of the internal combustion engine. Thecompression ratio therefore “oscillates” continuously around the knocklimit or stays below it. Because the compression ratio therefore iscontinuously being raised and lowered in the area of the knock limit,the maximum possible compression ratio is always obtained, no matterwhat the operating state of the internal combustion engine. It isadvantageous in this method that only the measurement values of theknock sensor need to be evaluated, which means that complicatedcombustion monitoring sensors inside the combustion chamber are notrequired. In addition, the effort which must be make to implement theclosed-loop or open-loop control of the system is simplified, becauseonly the signal of the knock sensor must be processed.

SUMMARY OF THE INVENTION

It is provided in accordance with a further elaboration that the firstvalue and/or the first time period is determined as a function of theload demand on the internal combustion engine. The first value and/orthe first time period will be different, depending on the load range inwhich the internal combustion engine is operating, so that, overall, thesawtooth course of the compression ratio curve will be composed ofsteeper or flatter sections, when seen on the coordinate system.

If, therefore, it can be recognized under dynamic operating conditions,that is, under conditions of changing loads, that the compression ratiomust be decreased quickly as a result of a sudden high load demand, thefirst value and/or the first time period and/or the second value and/orthe second time period must be changed in order to meet the demand for arapid decrease in compression. It can therefore be provided that thefirst value is increased and/or the first time period decreased in orderto achieve a rapid decrease in compression, which will then be followedby an increase in compression. If, for example, the knock sensorresponds again within, especially at the end of, the first time period,the first value by which the compression is decreased is increased, andit keeps being increased until the knock sensor no longer respondswithin the first time period. Then the first value is set back to itsbaseline value, which depends on the operating state.

If, under dynamic operating conditions, the compression ratio is to beincreased quickly as a result of a sudden drop in the load demand, thesecond value by which the compression is increased within the secondtime period may possibly not be sufficient to meet the demand for arapid increase in compression. If, therefore, the knock sensor does notrespond within, especially at the end of, the second time period, thecompression increase is repeated at a higher second value until theknock sensor starts responding again as the compress ratio increases.This leads again to a decrease in the second value of the compressionincrease to the baseline value associated with the operating state ofthe internal combustion engine.

In another elaboration of the invention, it is provided that operatingparameters of the internal combustion engine and/or of a motor vehicledriven by the internal combustion engine are taken into consideration inthe determination of the first value, of the first time period, of thesecond value, and/or of the second time period. Such parameters mightinclude, for example, the rpm's of the engine; the amount of fuelinjected; the position of the pedal sensor; the temperature of the air,coolant, and/or oil; the temperatures of the exhaust gas and/orcatalyst; the ambient air pressure; the charging pressure; the flow rateof the intake air; the driving speed; and/or the ignition timing.

The invention is especially applicable with advantage todirect-injection engines, especially to spark-ignition engines. Theinvention is also applicable especially to internal combustion engines,preferably engines for motor vehicles, which are equipped with NO_(x)storage catalysts.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is explained in greater detail on the basis of anexemplary embodiment with reference to the drawing. The single FIGUREshows the compression ratio versus time on a two-axis, orthogonalcoordinate system.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The FIGURE shows the compression ratio V versus the time t. It can beseen that the time change in the compression ratio V appears as asawtooth-like curve K. That is, there are sections of the curve with apreferably constant positive slope and others with a preferably constantnegative slope. The individual segments of the curve K can thereforealso be formed by straight lines.

In the diagram shown in the figure, a dotted line L is also shown, whichrepresents, as a curve, the knock limit, which depends on the operatingstate of the internal combustion engine (not shown). The line Ltherefore indicates the knock limit at which the knock sensor (not shownhere) responds.

Beginning at an arbitrary starting value t₀, the knock sensor determinesthat the compression ratio V is below the knock limit and that goodcombustion is thus present. Therefore, in a first time period t₁-t₀, thecompression ratio V is increased by a operating state-dependent valueV1, so that a section of the curve K has a constant, positive slope.When, within the time period t₁-t₀, the knock limit is reached, that is,when the compression ratio V exceeds the knock limit L, the compressionratio V is reduced in the following, second time period t₂-t₁ by asecond value V2, so that therefore a section of the curve K with aconstant negative slope is produced. Following the second time periodt₂-t₁ there is again a first time period t₃-t₂, during which thecompression ratio V is again increased by an operating point-dependentvalue, as also done during the time period t₁-t₀.

It can be seen from the figure that, starting at time t₂, first andsecond time periods follow in alternation, and thus that curve sectionsare present which have different slopes, that is, sections where thequotients of ΔV and Δt are different, the exact value of the slope beingchanged in particular as a function of the operating state of theengine.

Purely as an example, let it be assumed for the course of thecompression ratio V that, during the period t_(A)-t₀, a medium loaddemand is made on the engine and that, during the period t_(B)-t_(A),the engine is operating in response to a high load demand. The full-loadrange is at the lowest point of the curve. During the periodt_(C)-t_(B), the internal combustion engine is operating in the low loadrange.

During the first time period t_(n+1)-t_(n), the compression ratio V israised by a first value V1′. It can be seen that, at time t_(n+1), theknock limit L has still not been reached. Then, in the time periodt_(n+2)-t_(n+1), the value by which the compression ratio V rises isincreased by the value V1″; in addition, the time period t_(n+2)-t_(n+1)is shorter than the first time period t_(n+1)-t_(n). The slope in thissection of the curve is therefore made steeper. At the end of thissection, i.e., at time t_(n+2), the compression ratio V just reaches theknock limit. Then the compression is decreased again by a second,operating point-dependent value over a second time period.

So that it is possible to adapt or change the compression ratio V, it isprovided that the time at which the intake value of the combustionchamber of the engine closes is changed. If the compression is to bedecreased, this point in time is shifted in the “later” direction fromthe baseline point. To increase the compression, the time at which theintake valve is closed is shifted in the “earlier” direction. That is,the closing time of the intake valve is changed as a function of theresponse of the knock sensor. Thus it is possible, especially in theupper load range, which is especially prone to knocking, to achieve thehighest possible compression ratio at all times, so that the efficiencyof the internal combustion engine can be favorable influenced.

What is claimed is:
 1. A method for operating an internal combustionengine with at least one working piston with a variable compressionratio, which can be changed as a function of an operating state of theinternal combustion engine, the method comprising the steps of:detecting, with a knock sensor, when the compression ratio is too high;reducing the compression ratio (V) when the knock limit (L) is at leastreached by a first, predetermined value (V1), which is a function of theoperating state, over a first, predetermined time period (t₁-t₀);subsequently increasing the compression ratio (V) by a second,predetermined value (V2) over a second, predetermined time period(t₂-t₁) until the knock limit (L) is at least reached again; anddetermining at least one of the first predetermined value (V1), thefirst time period (t₁-t₀), the second predetermined value (V2), and thesecond time period (t₂-t₁) as a function of load demand on the internalcombustion engine.
 2. A method according to claim 1, including takingoperating parameters of at least one of the internal combustion engineand a motor vehicle driven by the internal combustion engine intoconsideration in determining at least one of the first value, the firsttime period, the second value, and the second time period.
 3. A methodaccording to claim 1, including lowering the compression ratio (V) by anincreased second value when the knock limit (L) is at least reachedwithin the second predetermined time period.
 4. A method according toclaim 3, wherein the compression ratio (V) is lowered by the increasedsecond valve when the knock limit (L) is reached at an end of the secondpredetermined time period.
 5. A method according to claim 1, includingraising the compression ratio (V) by an increased first value (V1′) whenthe compression ratio reaches or falls below the knock limit within thefirst predetermined time period.
 6. A method according to claim 5,wherein the compression ratio (V) is raised when the compression ratioreaches the knock limit at an end of the first time period.